High resolution imaging of the interior of the body (or for dermatologic or ophthalmology applications not restricted to the interior) serves multiple purposes, including any of i) assessing tissue structures, anatomy and composition; ii) planning and/or guiding interventions on localized regions of the body; and iii) assessing the result of interventions that alter the structure, composition or other properties of the localized region. High resolution imaging in this particular case refers to high frequency ultrasound and optical imaging methods. For the purposes of this invention, high frequency ultrasound typically refers to imaging with frequencies of greater than 3 MHz, and more typically in the range of 9 to 100 MHz.
High frequency ultrasound is very useful for intravascular and intracardiac procedures. For these applications, the ultrasound transducers are incorporated into a catheter or other device that can be inserted into the body. By way of example, two particularly important implementations of high frequency ultrasound are intravascular ultrasound (IVUS), for imaging blood vessels, and intracardiac echocardiography (ICE) for imaging cardiac chambers. Both ICE and IVUS are minimally invasive, and involve placing one or more ultrasound transducers inside a blood vessel or cardiac chamber to take high quality images of these structures.
Optical imaging methods based on fiber optic technology used in the field of medicine include optical coherence tomography (OCT), angioscopy, near infrared spectroscopy, Raman spectroscopy and fluorescence spectroscopy. These modalities typically require the use of one or more optical fibers to transmit light energy along a shaft between an imaging site and an imaging detector. Optical coherence tomography is an optical analog of ultrasound, and provides imaging resolutions on the order of 1 to 30 microns, but does not penetrate as deeply into tissue as ultrasound in most cases. Fiber optics can also be used to deliver energy for therapeutic maneuvers such as laser ablation of tissue and photodynamic therapy.
Additional forms of imaging related to this invention include angioscopy, endoscopy and other similar imaging mechanisms that involves imaging a site inside the patient using a probe to take pictures based on the back-reflection of light.
High resolution imaging means have been implemented in many forms for assessing several different regions of mammalian anatomy, including the gastrointestinal system, the cardiovascular system (including coronary, peripheral and neurological vasculature), skin, eyes (including the retina), the genitourinary systems, breast tissue, liver tissue and many others. By way of example, imaging of the cardiovascular system with high frequency ultrasound or optical coherence tomography has been developed for assessing the structure and composition of arterial plaque.
High-resolution imaging has been used to measure vessel or plaque geometry, blood flow through diseased arteries, the effects of interventions on arterial plaque (such as by atherectomy, angioplasty and/or stenting). Attempts have also been made using high resolution imaging to identify vascular lesions that have not led to clinical symptoms, but are at increased risk of rupturing or eroding and causing an acute myocardial infarction. These so-called “vulnerable plaques” are an area of interest as the prospect of treating such plaques to pre-empt adverse clinical events is conceptually appealing.
Chronic total occlusions are a specific subset of vascular lesions where the entire lumen of the vessel has been occluded (based on the angiographic appearance of the lesion) for over approximately one month. Most intravascular imaging modalities are “side-viewing” and require passage of an intravascular imaging device through a lesion. In order to image chronic total occlusions, methods of high resolution imaging would be more useful if they were adapted to a “forward-looking” rather than “side-viewing” configuration.
Several of these high resolution imaging means are dependent on the use of a rotary shaft to transmit torque to an imaging device near the distal end of the probe. These rotary shafts are often long, thin and flexible, such that they can be delivered through anatomical conduits, such as the vasculature, genitourinary tracts, respiratory tracts and other such bodily lumens. Ideally, when a continuous torque is applied to the cable in a specified direction the torque cable develops a property of having a close relation between the degree of rotation at its proximal and distal ends. This allows the simplification of the design of the ultrasound catheter by making the angle of rotation at the distal end of the torque cable (within the body) a reasonable approximation of the angle of rotation at the proximal end of the torque cable (outside of the body).
The rotation of the torque cable or shaft at the point from which the imaging occurs may not be identical to the rotation occurs at the proximal end of the torque cable or shaft. This occurs especially when the flexible shaft is delivered through tortuous passageways and is, at least in part, due to inertia and friction between the rotating components and stationary components of the imaging shaft. The assumption that the rotational speed of the proximal and distal ends of the rotary shaft are equal to each other is also less likely to be valid if the rotational speed varies over time. The undesirable result of not knowing the true angular velocity of the imaging probe at the point from which the imaging beam is directed towards the tissue leads to an artifact referred to non-uniform rotational distortion (NURD). NURD can lead to significant distortion of the image and a concomitant reduction in the geometric accuracy of the image. Knowledge of a more precise estimation of the true rotary speed of the distal rotary shaft or an imaging assembly attached to the rotary shaft can help overcome such distortion by providing more accurate information for image reconstruction. A better estimation of the rotary speed can also help improve the accuracy of co-registration of images when more than one imaging modality is implemented on the imaging probe (such as combined ultrasound and optical imaging).
While use of more than one type of imaging technique, such ultrasound and optical techniques, have both proved valuable in medical applications for high resolution imaging, they are not commonly used in tandem. As described in the Summary of the related art below, there are some designs that exist for the combination of optical and ultrasound technologies. However, the limitations in these designs have prevented their acceptance.
Namely, designs that incorporate optical and ultrasound technologies offset the ultrasound and optical imaging mechanisms, such as disclosed in (Maschke, U.S. Pat. No. 7,289,842 resulting in the acquisition of unaligned ultrasound and optical signals. Alignment of the resultant data from these two imaging means requires movement of the imaging mechanisms and is prone to registration errors due to (i) non-uniform rotational distortion (NURD), (ii) motion of the object occurring between successive imaging of the same location using the two imaging means, (iii) variability in the object being imaged, and (iv) difficulty in accurately tracking the location of the imaging means. All these effects result in inaccurate co-registration which limits the usefulness of the acquisition of data from the two imaging means.